1. Field of the Invention
The present invention relates to an improvement in optical instruments having a function to detect a visual axis, such as cameras, which calculate information on the user's visual axis from information on a rotational angle of the eyeball and stored parameters and which use this visual-axis information, for example, for selection of a focus-detecting point.
2. Related Background Art
There are provided a variety of conventional apparatus for detecting where a user is observing on an observation plane, i.e., for detecting the so-called line of sight (visual axis) (for example, eye cameras).
For example, Japanese Laid-open Patent Application No. 1-274736 discloses such technology that parallel beams from a light source are projected onto a front eye part of the user's eyeball and that the visual axis is obtained by utilizing positions of a corneal reflection image by reflected light from the cornea and an image of the pupil.
FIG. 1 is a drawing to illustrate the principle of the method for detecting the visual axis.
FIG. 2A shows an ordinary eyeball image projected on a surface of an image sensor 14 in FIG. 1, and numeral 60 in FIG. 2B indicates an image signal output along the line (E)-(E') in FIG. 2A.
In FIG. 2A, reference numeral 50 shows the white portion of the eye, 51 the pupil, and 52a, 52b corneal reflection images of light sources for illuminating the eyeball.
The visual-axis detecting method is described below using these FIG. 1 and FIGS. 2A, 2B.
Infrared light emitting diodes 13a, 13b are arranged nearly in symmetry with each other in the x direction with respect to the optical axis u of a light-receiving lens 12, each divergently illuminating the user's eyeball.
Infrared light emitted from the infrared light emitting diode 13b illuminates a cornea 16 of eyeball 15. On this occasion, a corneal reflection image d by part of infrared light reflected on the surface of the cornea 16 is condensed by the light-receiving lens 12 to be re-imaged at a position d' on the image sensor 14.
Similarly, infrared light emitted from the infrared light emitting diode 13a illuminates the cornea 16 of eyeball 15. On this occasion, a corneal reflection image e by part of infrared light reflected by the surface of the cornea 16 is condensed by the light-receiving lens 12 to be re-imaged at a position e' on the image sensor 14.
Also, beams from the edges a, b of the iris 17 are focused by the light-receiving lens 12 to form respective images of the edges a, b at unrepresented positions a', b' on the image sensor 14. Whenever the rotational angle .theta. of an optical axis v of the eyeball 15 relative to the optical axis u of the light-receiving lens 12 is small, letting xa and xb be x coordinates of the edges a, b of the iris 17, a coordinate position xc of the center position c of the pupil 19 is expressed as follows. EQU xc.perspectiveto.(xa+xb)/2
Since the x coordinate of the middle point between the corneal reflection images d and e is coincident with the x coordinate xo of the center of curvature o of the cornea 16, the rotational angle .theta. of the optical axis v of the eyeball 15 approximately satisfies the following relation of formula (1): EQU (A1*L.sub.oc)*sin .theta..perspectiveto.xc-(xd+xe)/2 (1)
where xd, xe are x coordinates of originating positions of the corneal reflection images d, e, respectively, L.sub.oc is a standard distance between the center of curvature o of the cornea 16 and the center c of the pupil 19, and A1 is a coefficient taking individual variations in distance L.sub.oc into account. Thus, a visual-axis processing unit can obtain the rotational angle .theta. of the optical axis v of the eyeball 15 by detecting positions of the characteristic points (corneal reflection images d, e and edges a, b of the iris 17) projected on part of image sensor 14. In this case, the above formula (1) can be rewritten as follows . EQU .beta.(A1*L.sub.oc)*sin .theta..perspectiveto.(xa'+xb')/2-(xd'+xe')/2(2)
Here, .beta. is a magnification determined by the position of the eyeball 15 relative to the light-receiving lens 12, which is obtained substantially as a function of a distance .vertline.xd'-xe'.vertline. between the corneal reflection images of the infrared light emitting diodes 13a, 13b. The rotational angle .theta. of the optical axis v of the eyeball 15 can be rewritten as follows. EQU .theta..perspectiveto.ARCSIN{(xc'-xf')/.beta./(A1*L.sub.oc)}(3)
Here, EQU xc'.perspectiveto.(xa'+xb')/2 EQU xf'.perspectiveto.(xd'+xe')/2.
Incidentally, the visual axis of the user's eyeball 15 does not coincide with the optical axis v in many cases, and, therefore, once the horizontal rotational angle .theta. of the optical axis v is calculated, the horizontal visual axis .theta.H of the user can be obtained by performing correction of angle .delta. between the optical axis v and the visual axis. Letting B1 be a coefficient taking into account individual variations in correction angle .delta. between the optical axis v and the visual axis of the eyeball 15, the horizontal visual axis .theta.H of the user can be obtained as follows. EQU .theta.H=.theta..+-.(B1*.delta.) (4)
Here, when clockwise rotation with respect to the user is defined as positive, the signs .+-. are determined in such a manner that the sign is + when the user's eye looking into the observation apparatus is the left eye and--when it is the right eye.
FIG. 1 shows an example where the user's eye rotates in the z-x plane (for example, the horizontal plane). Also, the visual axis can similarly be detected when the user's eye rotates in the z-y plane (for example, the vertical plane). It should be, however, noted that the vertical visual axis .theta.V is given as follows because the vertical component of the user's visual axis coincides with the vertical component .theta.' of the optical axis v of the eyeball 15, EQU .theta.V=.theta.'
Further, from the visual axis data .theta.H, .theta.V, the following gives positions (xn, yn) on a focusing screen in the finder view field observed by the user. ##EQU1## Here, m is a constant determined by the finder optical system in camera.
Here, the values of coefficients A1, B1 for correcting the individual variations of user's (photographer's) eyeball 15 are obtained in such a manner that, while keeping the user's eye fixed on a target provided at a predetermined position in the finder of camera, the values are obtained so as to make a position of a fixation point calculated according to the above formula (5) coincident with the position of the target.
This operation for attaining the coefficients A1, B1 for correcting the individual variations for each user as described above will be called as "calibration."
FIG. 3 shows an indication in the finder, in which 200 to 204 represent focus-detecting areas (distance-measuring points), 210, 211, 212 photometry areas in the finder, i.e., left area, center area, and right area, and 213 a peripheral area. Further, 23 denotes a field-of-view mask, and 24 an in-finder LCD (Liquid Crystal Display), which is a display portion for displaying various information (shutter time, etc.) of camera.
Here is described the above "calibration" operation, using FIG. 3.
First keeping the user's eye fixed at the rightmost end focus-detecting area 204, signals of an image of the user's eye at this moment are obtained to calculate the rotational angle of the eyeball thereby and then to store it. Next keeping the user's eye fixed at the left most end focus-detecting area 200, the rotational angle of the eyeball at this moment is similarly calculated and the calculated angle is stored. Since x, y coordinates of the respective focus-detecting areas 204, 200 on the finder are known, the two unknowns A1, B1 are calculated using the formula (5) from the two observation information.
Normally, the cameras having the visual-axis detecting apparatus have an independent mode called a "calibration mode," and the user is requested to execute the "calibration" in this mode, prior to execution of the visual axis detection.
Although the correction coefficients characteristic of each user can be attained simply by executing the above "calibration" operation, this arrangement requires new "calibration" for every change of user, which forces the user to perform such an extremely troublesome operation.